In power supplies designed to provide a constant voltage to a load circuit, it is desirable to minimize heat dissipation and to reduce power consumption. These goals, however, for power supplies operated off an AC line, are constrained by the requirement that the power supply must function properly for the lowest AC line voltage condition in conjunction with the largest power demand by the load circuit.
When the AC line voltage applied to the input of the power supply increases, the power supply becomes inefficient in its use of power from the line. Power which is not required by the load circuit must be dissipated to maintain a constant load circuit voltage. The same holds true when the power demanded by the load circuit reduces. In this case also, to maintain the proper constant load circuit voltage, power which is not required must be dissipated to maintain the constant load circuit voltage.
Thus, the power supply operates at peak efficiency only for the lowest AC line voltage with largest load circuit power demand which the supply must service.
Power utilization inefficiency can become particularly severe because many power supplies must be designed to function during brown out conditions where the AC line voltage is reduced for extended periods of time. The inefficiency can also be severe where the load circuit power demands continually fluctuate as operating conditions in the load circuit vary. These varying load circuit power demands can, for example, involve operation of motors, solenoids and other power consuming components which may continuously cycle on and off.